I'm an American and it definitely seems like we are in a significant and worsening loneliness crisis. I have no idea to what degree any of it is unique to Americans. Social connectedness, socialization rates, and companionship have all been declining for quite a while now. Lot's of potential causes and theories about it. [1] is a decent overview.
Like personally I'm doing great, and so are a lot of people I know, and I'm sure you as well. But I think a lot of Americans are struggling badly with their social lives.
#1 Reason is likely the urban fabric of places being non-walkable & car dependent. It's a physical structure that doesn't lead itself to spontaneity and new connections.
I recommend reading Robert Putnam's Bowling Alone (2000). American civic and social engagement has been declining in almost every measurable way for 50 years now. There is no way to deny this.
The largest contributor according the book's surveys and studies (and I love saying this) is television outcompeting in-person fun. Car dependency is a factor, but IIRC was factor #2 or #3. While this ranking was true at the time of publication, I would wager that time spent on "screens" is likely factor #1, #2, and #3 now.
I would wager that many people are fleeing their hometowns to socialize in cities not because they're walkable, but because the density of people increases, allowing you to have better odds meeting real humans who haven't been lost to the allure of the indoors.
Like personally I'm doing great, and so are a lot of people I know, and I'm sure you as well. But I think a lot of Americans are struggling badly with their social lives.
[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC9811250/